Ceramics generally excel in heat resistance, thermal impact strength, mechanical strength at high temperatures, wear resistance or insulating properties, and are best suited for use in parts or members used under severe conditions. However, there is indeed a little chance of using ceramics alone. For instance, in mechanical parts, ceramics is used in combination with other materials such as metals to fulfill the desired function.
However, since it is difficult to bond ceramics directly to metal, those skilled in the art have relied upon the process which involves metallizing the surface of ceramics by certain techniques and, thereafter, bonding a metal piece to the metallized surface of ceramics. To this end, use has been made of: the high-melting point metal technique in which a metallized paste is deposited onto the surface of ceramics by screen printing and, thereafter, the resulting product is heated in a non-oxidizing atmosphere; the active technique method in which a highly active metal (Ti, Zr) is placed on the surface of ceramics, and the whole is heated in a vacuum vessel or in an inert atmosphere; and the physical deposition technique in which a metal is ignited or heated intensely in vacuo, and the resulting vapor is deposited onto the surface of ceramics.
However, limitation has been put upon the type of ceramics to be deposited with a metal layer in the case of merely laminating such a layer onto ceramics, for example, in the case of the high-melting point metal technique using molybdenum and manganese. For instance, when the ceramic is silicon nitride, the amount of oxides contained in the ceramic, capable of forming a reaction phase with molybdenum and manganese, is so small that the metal-to-ceramics bonding strength becomes insufficient. The active metal technique has the same drawback as the high-melting point metal technique. The physical deposition technique is also disadvantageous in that, when other metallic member is bonded to the deposited metal layer by high-temperature brazing such as silver brazing, thermal diffusion takes place depending upon the type of metal to be deposited, resulting in a lowering of the bonding strength of the deposited metal with ceramics.
When the ceramic is zirconia, it is expected that sufficiently high metal-to-ceramics bonding strength is obtained by the use of the high-melting point metal technique, since zirconia is an oxide having a thermal expansion coefficient greater than the aforesaid silicon nitride. In some cases, however, insufficient bonding strength may be achieved owing to a difference in the composition of ceramics. Even with a metal-ceramic combination having a sufficient bonding strength, there is a problem in connection with the reliability of that bonding strength, when it is subjected to repeated thermal stress.
Furthermore, when other metallic member is bonded to the aforesaid metal layer by, e.g., brazing, distortions appear depending upon the type of the metallic member to be used, with the result that the durability of the product still decreases as a whole. This is also true of the cases where other types of ceramics are bonded to the aforesaid metal film.
It thus has been required that a metal film be laminated onto the surface of ceramics with high bonding strength to bond a ceramic member to a metallic member or other ceramic member by, e.g, brazing. Nonetheless, any existing products fail to stand up to use under every possible circumstances due to the low reactivity of ceramics to a film-forming metal and the difference in the coefficient of thermal expansion therebetween.